1. Field of the Invention
The present invention relates to orthopedic components, and, particularly, to intramedullary nails.
2. Description of the Related Art
Intramedullary nails may be used to align and stabilize fractures of a long bone, such as a femur. In a fractured femur, an intramedullary nail may be inserted into the intramedullary canal of the femur and positioned to extend across the fracture line of the femur. Then, screws or other securement devices may be inserted through bores formed in the intramedullary nail on opposing sides of the fractured femur to secure the opposing portions of the fractured femur together.
If the head and/or neck of a long bone, such as the head and/or neck of the femur, has fractured, a lag screw may be inserted into a transverse bore formed in the intramedullary nail. This bore is aligned so that the lag screw extends through the neck and into the head of the long bone and across the fracture line, allowing the lag screw to reduce the fracture of the neck and/or head of the long bone.
For example, referring to FIG. 1, femur 10 is shown including shaft 12, neck 14, and head 16. As shown, neck 14 of femur 10 has been fractured at line 17. Transverse bore 18 extends through intramedullary nail 20 and is sized to receive lag screw 22 therethrough. Specifically, lag screw 22 has an outer diameter that is slightly smaller than the diameter of transverse bore 18. This allows lag screw 22 to pass through transverse bore 18 and reduce the fracture at line 17.
However, due to the need for lag screw 22 to have an outer diameter that is less than the diameter of transverse bore 18, lag screw 22 will pivot slightly within transverse bore 18 of intramedullary nail 20 when a force is applied to the end of lag screw 22. For example, force FG may be exerted on the end of lag screw 22, which results from head 16 of femur 10 bearing the weight of an individual. When force FG is applied to the end of lag screw 22, lag screw 22 pivots slightly within transverse bore 18 of intramedullary nail 20 to create two support points that bear the resultant forces. First support point 24 is a medial, distal support point, where force FM acts on lag screw 22, and second support point 26 is a lateral, proximal support point, where force FL acts on lag screw 22. By exerting a force on first and second support points 22, 24, force FM induces a compressive stress in the mass of the lower part of intramedullary nail 20, while force FL induces a tensile stress in the region of transverse bore 18. Additionally, force FL acting on support point 26 is amplified by the leverage ratio of lag screw 22 within transverse bore 18. The resulting, theoretical stress distribution is shown in FIG. 3, where the stress is concentrated around the medial and lateral openings of transverse bore 18.
Referring to FIG. 2, which shows a cross-section of intramedullary nail 20, the maximum tension caused by force FL is found near the lateral opening of transverse bore 18, which has sharp edges 27 that are formed in a region with very critical geometry. In addition to the maximum tension occurring at the lateral most side of intramedullary nail 10, the formation of transverse bore 18 in intramedullary nail 20 creates a notch effect that further concentrates stress along sharp edges 27 of the lateral opening of transverse bore 18, where a minimal amount of material is provided. Specifically, the region about the lateral opening of transverse bore 18, for example, has a minimal amount of material positioned thereabove as a result of the shape of intramedullary nail 20. Stated another way, because intramedullary nail 20 has a substantially circular cross-section in a direction perpendicular to the longitudinal axis of intramedullary nail 20 and because the lateral opening to transverse bore 18 is located at an outer edge of intramedullary nail 20, a minimal amount of material is provided in the region of support point 26 and the lateral opening of transverse bore 18, as compared to the amount of material in the region closer to the longitudinal axis of intramedullary nail 20. As a result of having a minimal amount of material in the region of the lateral opening of transverse bore 18, the material in the region of the lateral opening of transverse bore 18 has a greater concentration of stress than the material that is closer to the longitudinal axis of intramedullary nail 20. This requires that intramedullary nail 20 is formed from stronger, more expensive materials in order to withstand the increased concentration of stress in the material adjacent to the lateral opening of transverse bore 18 and/or has an increased size in the region of intramedullary nail 20 near the lateral opening of transverse bore 18 in order to increase the volume of material present and to decrease the concentration of stress adjacent to the lateral opening of transverse bore 18.